The invention relates to a filter element for use in a unit to filter and purify liquids. In particular, the invention is directed to a filter element used in a purification unit wherein a layer of finely divided particles of a suitable filter medium is precoated upon several tubular filter elements, housed within a tank such as that shown and disclosed in Ryan U.S. Pat. No. 3,373,104 which is assigned to the assignee of this application. The medium of finely divided particles deposited upon the filter element serves one or both of the following functions: first, to reduce the content of undissolved solids, such as heavy metal oxide, colloidal matter, and silt, with an efficiency of greater than 90%; and second, to reduce the level of dissolved impurities, such as dissolved salts, from an influent level of greater than 50 ppb (parts per billion) to less than 10 ppb.
It is known that in order to provide efficient and effective removal of dissolved and undissolved impurities by means of such a purification unit, uniform distribution of precoat and subsequent uniform distribution of influent liquid over the length of the precoated tubular filter element is desirable. Furthermore, in order to achieve uniform and complete backwashing of the precoat from the filter element after a service cycle termination point has been reached, uniform distribution of backwash liquid along the length of the tubular element is also essential. In both the service and backwash cycle directions, the liquid tends to avoid distribution by seeking a path offering the lowest pressure drop. Therefore, flow into or out of the element is normally greater near its discharge end. The use of relatively long tubular filter elements having a length of approximately 60 inches and greater with a diameter of approximately 2 inches increases the problem encountered in distributing both service cycle influent and backwash liquid uniformly over the length of the tubular filter elements.
Previously known apparatus have been devised to distribute not only both a filter precoat and an influent liquid but also a backwash liquid along the length of a filter element. Once such tubular filter element with a distributor core is disclosed in Quinlan U.S. Pat. No. 3,253,714. The distributor element disclosed in Quinlan is a centrally-located tube having a plurality of liquid-passage holes or apertures distributed over its wall. The total cross-sectional area of all of these holes is significantly less than the cross-sectional area of the interior of the central tube transverse to its axis. The ratio of these cross-sectional areas taught in Quinlan is that the total of the cross-sectional area of all of the liquid passages holes in the central tube should be about two-thirds of the tube's interior transverse cross-sectional area, and can vary from about one-third to about eight-tenths of that area. These ratios have proved to be inefficient in distributing liquid along the tubular filter elements having a high length-to-diameter ratio. For a tube having a diameter of one inch or less, the relatively large pressure loss effects of elevation differences along the filter element and friction due to flow in the central tube cause difficulties in obtaining even distribution.
Furthermore, tubular filter elements of the prior art have generally includes an unobstructed annular passage zone between the central distributor tube and a filter medium retaining means. This unobstructed annular zone extending for the length of the filter element has allowed liquids to continue to seek the portion of the filter element having the lowest pressure drop along its length both during the service and backwash cycles. Such an unobstructed annular passageway continues the shortcomings of non-uniform deposition of filter precoat over the entire length of the tubular filter element, non-uniform exhaustion of the dissolved and undissolved solids-removal capacity of the filter precoat along the length of the tubular element, and inefficient removal of the filter precoat during backwash, particularly near the uppermost end of the tubular filter element.
When circular seals have been provided around the central tubes, as in Woodruff U.S. Pat. No. 2,796,939, an annular space between the central tube and an outer screen has been filled with a filter medium, such as gravel. This volume of gravel restricts the unimpeded distribution of backwash liquid throughout the annular chamber formed between adjacent circular seals. Also, the gravel filter medium is close to apertures in the central tube so that backwash liquid cleans only the area of the filter medium close to the apertures. Circular seals in the prior art also have the disadvantage of being positioned at equal distances along the length of the filter element, thereby requiring relatively high velocities of backwash liquid and small apertures in the distributor core, which are prone to plugging and difficult to form inexpensively, to provide the high pressure drops needed to accomplish uniform distribution along the length of the filter element.